New SSD for Asus Zenbook UX32A

Hi guys
Im looking for new SSD replacement of stock HDD in my Asus UX32A (core i5 modification, with 24gb ssd, 500gb hdd, 4gb ram). My choice fell on samsung's 840 EVO but than i'm scanned my system, and found my HDD (hitachi connected through sata 2 (while ssd through sata 3 WAT?!) and now I doubt that the speed gain from SSD (sams has sata3) will be great.
Maybe i'm wrong, and my Hitachi works on SATA2 coz it supports only SATA2, and SSD will be working with SATA3??
I really appreciate your help, especially the help from the owners who have already did upgrade.